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The development of an optical fibre sensor for the measurement of dissolved oxygen based on sol-gel immobilisation technology and phase fluorimetric analysis is investigated. The sensor is based on indicator-mediated sensing where the optical properties of an intermediate reagent are modulated in response to the concentration of analyte. The fluorescent lifetime of an oxygen indicator, tris(1, 10-phenanthroline)ruthenium(II) chloride hydrate is shown to be quenched in the presence of dissolved oxygen. Changes in fluorescent lifetime are measured using phase modulation techniques, the phase shift in the fluorescence emission of the fluorophore is measured as a function of the dissolved oxygen concentration. Sol-gel technology is utilised to immobilise the indicator at the distal end of a multi-mode optical fibre using dip-coating techniques. The selection of the sol-gel precursor compound is found to be of critical importance in the performance of the sensor, with organically modified silicates displaying the best performance.